SERIAL HOMOLOGY OF THE VERTEBRAE. 21 



lower over the sharp edge below. These appear from the eighteenth to the twentieth year, 

 and are united about the twenty-fifth. The bodies of the sacral vertebrae are at first separated 

 by intervertebral discs, but about the eighteenth year, in the case of the lower vertebrae, 

 ossification begins to extend through these discs and the epiphyses, so as completely to unite 

 the adjacent bodies. The ossific union of the first and second bodies does not take place till the 

 twenty-fifth year or later. Previous to this, the lateral masses have coalesced in the same 

 order as the bodies. 



Coccygreal vertebrae. Each of the coccygeal vertebras is ossified from a distinct piece of 

 cartilage, and usually from a single centre, but in the upper sometimes from two centres. 

 Ossification commences in the first generally about the time of birth ; in the second, from the 

 fifth to the tenth year ; in the third, some time before, and in the fourth, some time after 

 puberty. The ossific union of the three lower coccygeal vertebras occurs before middle life ; 

 their union with the first, and the union of this with the sacrum, belong to the later periods 

 of life. 



Variations may occur in the mode of ossification of the vertebras, as is shown by the nature 

 of certain malformations. Thus, instances are recorded in which a vertebral body is 

 represented by two pieces of bone, separated by a median interval : here ossification must 

 have proceeded from two centres, as is the case normally in the odontoid process, and 

 sometimes in the body of the axis : probably this is the reappearance or persistence of a 

 primitive condition. Similarly, the neural arch of the fifth lumbar vertebra (very rarely of 

 one of the other vertebrae) may have a double origin, being divided by a cleft passing 

 obliquely between the superior and inferior articular processes : according to Rambaud and 

 Renault l there are always two nuclei in each half of the neural arch. 



SEBIAL HOMOLOGY OF THE VEBTEBItfflS. 



It is evident from the foregoing descriptions that the different segments of the vertebral 

 column are generally similar in construction ; and this similarity is for the most part so 

 marked that there can be no doubt of the homodynamy of the several constituent parts of 

 successive vertebras. In certain instances, however, there are peculiarities of arrangement 

 which offer some difficulty in tracing the precise correspondence, and in explanation of these 

 chiefly the following brief sketch of the serial homology of the vertebrae is given. The 

 accompanying views of the vertebrae and some of their varieties (fig. 23) will also assist the 

 reader in comparing their forms. 



1. The series of centra or bodies, surrounding the primary axis of the notochord, is 

 complete in man, from the odontoid process of the second vertebra to the caudal extremity. 

 It must, however, be remembered that the part called the body of a vertebra includes not only 

 the proper central ossification, but also the ventral extremity of the neural ossification on each 

 side. The body is apparently absent in the atlas, but the part corresponding to the central 

 ossification of that vertebra is united with the body of the axis in the odontoid process ; 

 while the anterior arch of the atlas probably belongs to the series of subcentral parts or 

 hypapopliyses. The proofs of this view are derived mainly from, 1st, the remains of the noto- 

 chord having been traced in the foetus through the odontoid process (and not through the 

 anterior arch of the atlas) into the base of the skull ; 2nd, the separate ossification in cartilage 

 of the odontoid process ; and 3rd, the existence in some animals, as the ornithorhynchus and 

 some reptiles, of a bone corresponding to the odontoid process, in a separate condition, without 

 any other part representing the centrum of the atlas. 



2. The series of neural arches is complete in the whole vertebral column of man, with 

 the exception of the lower three coccygeal vertebras, and in part of the first coccygeal and 

 lower sacral vertebras. The neural spines are also complete in nearly the same vertebras as 

 the arches. The spine is absent or little developed in the atlas, bifid at its extremity in the 

 next four or five cervical vertebrae, but simple in all the remaining vertebras in which it is 

 present. 



3. The articular processes or zygupophyses, superior and inferior (preaxial and 

 postaxial), correspond in their relations throughout the whole of the vertebrae in which they 

 exist, with the exception of both of those of the atlas and the superior of the axis. In these 

 vertebras the articular processes are not in the series of zygapophyses, being situated in front 

 of the place of exit of the spinal nerves, instead of behind it, and therefore in a position 

 which corresponds to that of the part of the vertebral body formed from the neural ossifica- 

 tion. The joints which they form must accordingly be looked upon as homologous with the 

 lateral portions of the articulations between the bodies of the succeeding vertebrae. In the 

 sacral vertebras the articular processes, existing as such in early life, come to be in the adult 

 united by anchylosis. In the lower three coccygeal vertebras they are absent. 



1 "Origine et Developpement des Os," Paris, 1864. This work may be referred to for more detailed 

 information concerning the ossification of the vertebrae, as well as of the bones generally. 



